THE ANCIENTLIFE-HISTORY
OF THE EARTH

Chapter 8:

THE CAMBRIAN PERIOD.

The traces of life in the Laurentian period, as we have seen, are
but scanty; but the Cambrian Rocks—so called from their
occurrence in North Wales and its borders ("Cambria ")—have
yielded numerous remains of animals and some dubious plants. The
Cambrian deposits have thus a special interest as being the oldest
rocks in which occur any number of well-preserved and unquestionable
organisms. We have here the remains of the first fauna, or
assemblage of animals, of which we have at present knowledge.
As regards their geographical distribution, the Cambrian Rocks
have been recognised in many parts of the world, but there is
some question as to the precise limits of the formation, and
we may consider that their most typical area is in South Wales,
where they have been carefully worked out, chiefly by Dr Henry
Hicks. In this region, in the neighbourhood of the promontory
of St David's, the Cambrian Rocks are largely developed, resting
upon an ancient ridge of Pre-Cambrian (Laurentian?) strata, and
overlaid by the lowest beds of the Lower Silurian. The subjoined
sketch-section (fig. 27) exhibits in a general manner the succession
of strata in this locality.

From this section it will be seen that the Cambrian Rocks in Wales
are divided in the first place into a lower and an upper group.
The Lower Cambrian is constituted at the base by a great
series of grits, sandstones, conglomerates, and slates, which
are known as the "Longmynd group," from their vast development in
the Longmynd Hills in Shropshire, and which attain in North Wales
a thickness of 8000 feet or more. The Longmynd beds are succeeded
by the so-called "Menevian group," a series of sandstones, flags,
and grits, about 600 feet in thickness, and containing a
considerable number of fossils. The Upper Cambrian series
consists in its lower portion of nearly 5000 feet of strata,
principally shaly and slaty, which are known as the "Lingula
Flags," from the great abundance in them of a shell referable
to the genus Lingula. These are followed by 1000 feet
of dark shales and flaggy sandstones, which are known as the
"Tremadoc slates," from their occurrence near Tremadoc in North
Wales; and these in turn are surmounted, apparently quite
conformably, by the basement beds of the Lower Silurian.

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GENERALIZED SECTION OF THE CAMBRIAN ROCKS IN WALES
Fig. 27.
The above may be regarded as giving a typical series of the Cambrian
Rocks in a typical locality; but strata of Cambrian age are known in
many other regions, of which it is only possible here to allude to
a few of the most important. In Scandinavia occurs a well-developed
series of Cambrian deposits, representing both the lower and upper
parts of the
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formation. In Bohemia, the
Upper Cambrian, in particular, is largely developed, and constitutes
the so-called "Primordial zone" of Barrande. Lastly, in North
America, whilst the Lower Cambrian is only imperfectly developed,
or is represented by the Huronian, the Upper Cambrian formation has
a wide extension, containing fossils similar in character to the
analogous strata in Europe, and known as the "Potsdam Sandstone."
The subjoined table shows the chief areas where Cambrian Rocks are
developed, and their general equivalency:

Like all the older Palæozoic deposits, the Cambrian Rocks,
though by no means necessarily what would be called actually
"metamorphic," have been highly cleaved, and otherwise altered
from their original condition. Owing partly to their indurated
state, and partly to their great antiquity, they are usually
found in the heart of mountainous districts, which have undergone
great disturbance, and have been subjected to an enormous amount
of denudation. In some cases, as in the Longmynd Hills in
Shropshire, they form low rounded elevations, largely covered by
pasture, and with few or no elements of sublimity. In other cases,
however, they rise into bold and rugged mountains, girded by
precipitous cliffs. Industrially, the Cambrian Rocks are of
interest, if only for the reason that the celebrated Welsh slates
of Llanberis are derived from highly-cleaved beds of this age.
Taken as a whole, the Cambrian formation is essentially composed
of arenaceous and
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muddy sediments, the
latter being sometimes red, but more commonly nearly black in
colour. It has often been supposed that the Cambrians are a
deep-sea deposit, and that we may thus account for the few
fossils contained in them; but the paucity of fossils is to a
large extent imaginary, and some of the Lower Cambrian beds
of the Longmynd Hills would appear to have been laid down in
shallow water; as they exhibit rain-prints, sun-cracks, and
ripple-marks—incontrovertible evidence of their having been
a shore-deposit. The occurrence, of innumerable worm-tracks and
burrows in many Cambrian strata is also a proof of shallow-water
conditions; and the general absence of limestones, coupled with
the coarse mechanical nature of many of the sediments of the
Lower Cambrian, maybe taken as pointing in the same direction.

The life of the Cambrian, though not so rich as in the
succeeding Silurian period, nevertheless consists of representatives
of most of the great classes of invertebrate animals. The coarse
sandy deposits of the formation, which abound more particularly
towards its lower part, naturally are to a large extent barren
of fossils; but the muddy sediments, when not too highly cleaved,
and especially towards the summit of the group, are replete with
organic remains. This is also the case, in many localities at any
rate, with the finer beds of the Potsdam Sandstone in America.
Limestones are known to occur in only a few areas (chiefly in
America), and this may account for the apparent total absence
of corals. It is, however, interesting to note that, with this
exception, almost all the other leading groups of Invertebrates
are known to have come into existence during the Cambrian period.

Of the land-surfaces of the Cambrian period we know nothing;
and there is, therefore, nothing surprising in the fact that
our acquaintance with the Cambrian vegetation is confined to
some marine plants or sea-weeds, often of a very obscure and
problematical nature. The "Fucoidal Sandstone" of Sweden, and the
"Potsdam Sandstone" of North America, have both yielded numerous
remains which have been regarded as markings left by sea-weeds or
"Fucoids;" but these are highly enigmatical in their characters,
and would, in many instances, seem to be rather referable to the
tracks and burrows of marine worms. The first-mentioned of these
formations has also yielded the curious, furrowed and striated
stems which have been described as a kind of land-plant under the
name of Eopkyton (fig. 28). It cannot be said, however,
that the vegetable origin of these singular bodies has been
satisfactorily proved. Lastly, there are found in certain green
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and purple beds of Lower Cambrian age at Bray Head, Wicklow,
Ireland, some very remarkable fossils, which are well known under
Fig. 28.—Fragment of Eophyton Linneanum, a supposed
land-plant. Lower Cambrian, Sweden, of the natural size.
the name of Oldhamia, but the true nature of which is
very doubtful. The commonest form of Oldhamia (fig. 29)
consists of a thread-like stem or axis, from which spring at
regular intervals bundles of short filamentous branches in a
fan-like manner. In the locality where it occurs, the fronds
of Oldhamia are very abundant, and are spread over the
surfaces of the strata in tangled layers. That it is organic
is certain, and that it is a calcareous sea-weed is probable;
but it may possibly belong to the sea-mosses (Polyzoa),
or to the sea-firs (Sertularians).

Amongst the lower forms of animal life (Protozoa), we
find the Sponges represented by the curious bodies, composed
of netted fibres, to which the name of Protospongia has
been given (fig. 32, a); and the comparatively gigantic,
conical, or
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cylindrical fossils termed Archœocyathus by Mr
Billings are certainly referable either to the Foraminifera
Fig. 29.—A portion of Oldhamia antiqua, Lower
Cambrian, Wicklow, Ireland, of the natural size. (After Salter.)
or to the Sponges. The almost total absence of limestones in
the formation may be regarded as a sufficient explanation of
the fact that the Foraminifera are not more largely and
unequivocally represented; though the existence of greensands
in the Cambrian beds of Wisconsin and Tennessee may be taken as
an indication that this class of animals was by no means wholly
wanting. The same fact may explain the total absence of corals,
so far as at present known.

The group of the Echinodermata (Sea-lilies, Sea-urchins,
and their allies) is represented by a few forms, which are
principally of interest as being the earliest-known examples of
the class. It is also worthy of note that these precursors of a
group which subsequently attains such geological importance, are
referable to no less than three distinct orders—the
Crinoids or Sea-lilies, represented by a species of
Dendrocrinus; the Cystideans by Protocystites; and
the Star-fishes by Palasterina and some other forms. Only
the last of these groups, however, appears to occur in the Lower
Cambrian.

The Ringed-worms (Annelida), if rightly credited with all
the remains usually referred to them, appear to have swarmed in
the Cambrian seas. Being soft-bodied, we do not find the actual
worms themselves in the fossil condition, but we have, nevertheless,
abundant traces of their existence. In some cases we find vertical
burrows of greater or less depth, often expanded towards their
apertures, in which the worm must have actually lived (fig. 30),
as various species do at the present day. In these cases, the
tube must have been rendered more or less permanent by receiving
a coating of mucus, or perhaps a genuine membranous secretion,
from the body of the animal; and it may be found quite empty, or
occupied by a cast of sand or mud. Of this nature are the burrows
which have been described under the names of Scolithus and
Scolecoderma, and probably the Histioderma of the
Lower Cambrian
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of Ireland. In other cases, as in Arenicolites
(fig. 32, b), the worm seems to have inhabited a double
Fig. 30.—Annelide-burrows (Scolithus linearus)
from the Potsdam Sandstone of Canada, of the natural size. (After
Billings.)
burrow, shaped like the letter U, and having two openings placed
close together on the surface of the stratum. Thousands of these
twin-burrows occur in some of the strata of the Longmynd, and
it is supposed that the worm used one opening to the burrow as
an aperture of entrance, and the other as one of exit. In other
cases, again, we find simply the meandering trails caused by the
worm dragging its body over the surface of the mud. Markings of
this kind are commoner in the Silurian Rocks, and it is generally
more or less doubtful whether they may not have been caused by
other marine animals, such as shellfish, whilst some of them
have certainly nothing whatever to do with the worms. Lastly,
the Cambrian beds often show twining cylindrical bodies, commonly
more or less matted together, and not confined to the surfaces
of the strata, but passing through them. These have often been
regarded as the remains of sea-weeds, but it is more probable
that they represent casts of the underground burrows of worms
of similar habits to the common lob-worm (Arenicola) of
the present day.

The Articulate animals are numerously represented in the
Cambrian deposits, but exclusively by the class of
Crustaceans. Some of these are little double-shelled
creatures, resembling our living water-fleas (Ostracoda). A
few are larger forms, and belong to the same group as the existing
brine-shrimps and fairy-shrimps (Phyllopoda). One of the
most characteristic
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of these is the
Hymenocaris vermicauda of the Lingula Flags (fig.
32, d). By far the larger number of the Cambrian
Crustacea belong, however, to the remarkable and wholly
extinct group of the Trilobites. These extraordinary
animals must have literally swarmed in the seas of the later
portion of this and the whole of the succeeding period; and they
survived in greatly diminished numbers till the earlier portion
of the Carboniferous period. They died out, however, wholly before
the close of the Palæozoic epoch, and we have no Crustaceans
at the present day which can be considered as their direct
representatives. They have, however, relationships of a more or
less intimate character with the existing groups of the Phyllopods,
the King-crabs (Limulus), and the Isopods ("Slaters,"
Wood-lice, &c.) Indeed, one member of the last-mentioned
order, namely, the Serolis of the coasts of Patagonia, has
been regarded as the nearest living ally of the Trilobites. Be
this as it may, the Trilobites possessed a skeleton which, though
capable of undergoing almost endless variations, was wonderfully
constant in its pattern of structure, and we may briefly describe
here the chief features of this.

The upper surface of the body of a Trilobite was defended by a
strong shell or "crust," partly horny and partly calcareous in
its composition. This shell (fig. 31) generally exhibits a very
distinct "trilobation" or division into three longitudinal lobes,
one central and two lateral. It also exhibits a more important and
more fundamental division into three transverse portions, which
are so loosely connected with one another as very commonly to be
found separate. The first and most anterior of these divisions
is a shield or buckler which covers the head; the second or middle
portion is composed of movable rings covering the trunk ("thorax
"); and the third is a shield which covers the tailor "abdomen."
The head-shield (fig. 31, e) is generally more or less
semicircular in shape; and its central portion, covering the
stomach of the animal, is usually strongly elevated, and generally
marked by lateral furrows. A little on each side of the head are
placed the eyes, which are generally crescentic in shape, and
resemble the eyes of insects and many existing Crustaceans in being
"compound," or made up of numerous simple eyes aggregated together.
So excellent is the state of preservation of many specimens of
Trilobites, that the numerous individual lenses of the eyes have
been uninjured, and as many as four hundred have been counted in
each eye of some forms. The eyes may be supported upon prominences,
but they are never carried on movable stalks (as they are in
the existing lobsters and crabs); and
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in some of the Cambrian
Trilobites, such as the little Agnosti (fig. 31 g),
the animal was blind. The lateral portions of the head-shield
are usually separated from the central portion by a peculiar
Fig. 31.—Cambrian Trilobites: a, Paradoxides
Bohemicus, reduced in size; b, Ellipsocephalus Hoffi; c,
Sao hirsuta; d, Conocorypke Sultzeri (all the above, together
with fig. g, are from the Upper Cambrian or "Primordial
Zone" of Bohemia); e, Head-shield of Dikellocephalus
Celticus, from the Lingula Flags of Wales; f, Head-shield
of Conocoryphe Matthewi, from the Upper Cambrian (Acadian
Group) of New Brunswick; g, Agnostus rex, Bohemia; h,
Tail-shield of Dikellocephalus Minnesotensis, from the
Upper Cambrian (Potsdam Sandstone) of Minnesota. (After Barrande,
Dawson, Salter, and Dale Owen.)
line of division (the so-called "facial suture") on each side;
but this is also wanting in some of the Cambrian species. The
backward angles of the head-shield, also, are often prolonged
into spines, which sometimes reach a great length. Following
the head-shield behind, we have a portion of the body which is
composed of movable segments or "body-rings," and which is
technically called the "thorax," Ordinarily, this region is
strongly trilobed, and each ring consists of a central convex
portion, and of two flatter side-lobes. The number of body-rings
in the thorax is very variable (from two to twenty-six), but is
fixed for the adult forms of each group of the Trilobites. The
young forms have much fewer rings than the full-grown ones; and
it is curious to find that the Cambrian
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Trilobites very commonly have either a great many rings (as in
Paradoxides, fig. 31, a), or else very few (as in
Agnostus, fig. 31, g). In some instances, the
body-rings do not seem to have been so constructed as to allow
of much movement, but in other cases this region of the body is
so flexible that the animal possessed the power of rolling itself
up completely, like a hedgehog; and many individuals have been
permanently preserved as fossils in this defensive condition.
Finally, the body of the Trilobite was completed by a tail-shield
(technically termed the "pygidium"), which varies much in size
and form, and is composed of a greater or less number of rings,
similar to those which form the thorax, but immovably
amalgamated with one another (fig. 31, h).

The under surface of the body in the Trilobites appears to have
been more or less entirely destitute of hard structures, with the
exception of a well-developed upper lip, in the form of a plate
attached to the inferior side of the head-shield in front. There
is no reason to doubt that the animal possessed legs; but these
structures seem to have resembled those of many living Crustaceans
in being quite soft and membranous. This, at any rate, seems to
have been generally the case; though structures which have been
regarded as legs have been detected on the under surface of one
of the larger species of Trilobites. There is also, at present,
no direct evidence that the Trilobites possessed the two pairs
of jointed feelers ("antennæ") which are so characteristic
of recent Crustaceans.

The Trilobites vary much in size, and the Cambrian formation
presents examples of both the largest and the smallest members
of the order. Some of the young forms may be little bigger than
a millet-seed, and some adult examples of the smaller species
(such as Agnostus) may be only a few lines in length;
whilst such giants of the order as Paradoxides and
Asaphus may reach a length of from one to two feet. Judging
from what we actually know as to the structure of the Trilobites,
and also from analogous recent forms, it would seem that these
ancient Crustaceans were mud-haunting creatures, denizens of
shallow seas, and affecting the soft silt of the bottom rather
than the clear water above. Whenever muddy sediments are found
in the Cambrian and Silurian formations, there we are tolerably
sure to find Trilobites, though they are by no means absolutely
wanting in limestones. They appear to have crawled out upon the
sea-bottom, or burrowed in the yielding mud, with the soft under
surface directed downwards; and it is probable that they really
derived their nutriment from the organic matter contained in the
ooze amongst which they
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lived. The vital
organs seem to have occupied the central lobe of the skeleton,
by which they were protected; and a series of delicate leaf-like
paddles, which probably served as respiratory organs, would appear
to have been carried on the under surface of the thorax. That
they had their enemies may be regarded as certain; but we have no
evidence that they were furnished with any offensive weapons, or,
indeed, with any means of defence beyond their hard crust, and
the power, possessed by so many of them, of rolling themselves
into a ball. An additional proof of the fact that they for the
most part crawled along the sea-bottom is found in the occurrence
of tracks and markings of various kinds, which can hardly be
ascribed to any other creatures with any show of probability.
That this is the true nature of some of the markings in question
cannot be doubted at all; and in other cases no explanation so
probable has yet been suggested. If, however, the tracks which
have been described from the Potsdam Sandstone of North America
under the name of Protichnites are really due to the
peregrinations of some Trilobite, they must have been produced
by one of the largest examples of the order.

As already said, the Cambrian Rocks are very rich in the remains
of Trilobites. In the lowest beds of the series (Longmynd Rocks),
representatives of some half-dozen genera have now been detected,
including the dwarf Agnostus and the giant Paradoxides.
In the higher beds, the number both of genera and species is
largely increased; and from the great comparative abundance of
individuals, the Trilobites have every right to be considered as
the most characteristic fossils of the Cambrian period,—the
more so as the Cambrian species belong to peculiar types, which,
for the most part, died out before the commencement of the Silurian
epoch.

All the remaining Cambrian fossils which demand any notice here
are members of one or other division of the great class of the
Mollusca, or "Shell-fish" properly so called. In the Lower
Cambrian Rocks the Lamp-shells (Brachiopoda) are the principal
or sole representatives of the class, and appear chiefly in three
interesting and important types—namely, Lingulella,
Discina, and Obolella. Of these the last (fig. 32,
i) is highly characteristic of these ancient deposits;
whilst Discina is one of those remarkable persistent types
which, commencing at this early period, has continued to be
represented by varying forms through all the intervening geological
formations up to the present day. Lingulella (fig. 32,
c), again, is closely allied to the existing "Goose-bill"
Lamp-shell (Lingula anatina), and thus presents us with
another example of an extremely long-lived
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type. The Lingulellœ and their successors; the
Lingulœ, are singular in possessing a shell which
is of a horny texture, and contains but a small proportion of
calcareous matter. In the Upper Cambrian Rocks, the
Lingulellœ become much more abundant, the broad
satchel-shaped species known as L. Davisii (fig. 32, e)
being so abundant that one of the great divisions of the Cambrian
is termed the "Lingula Flags." Here, also, we meet for the first
time with examples of the genus Orthis (fig. 32, f, k, l)
a characteristic Palæozoic type of the Brachiopods, which is
destined to undergo a vast extension in later ages.

Of the higher groups of the Mollusca the record is as yet
but scanty. In the Lower Cambrian, we have but the thin, fragile,
dagger-shaped shells of the free-swimming oceanic Molluscs or
"Winged-snails" (Pteropoda), of which the most characteristic
is the genus Theca (fig. 32, g). In the Upper Cambrian,
in addition to these, we have a few Univalves (Gasteropoda),
and, thanks to the researches of Dr Hicks, quite a small assemblage
of Bivalves (Lamellibranchiata), though these are mostly
of no great dimensions (fig. 32, h). Of the chambered
Cephalopoda (Cuttle-fishes and their allies), we have but
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few traces; and these wholly confined to the higher beds of the
formation. We meet, however, with examples of the wonderful genus
Orthoceras, with its straight, partitioned shell, which we
shall find in an immense variety of forms in the Silurian rocks.
Lastly, it is worthy of note that the lowest of all the groups of
the Mollusca—namely, that of the Sea-mats, Sea-mosses,
and Lace-corals (Polyzoa)—is only doubtfully known to
have any representatives in the Cambrian, though undergoing a
large and varied development in the Silurian deposits.

An exception, however, may with much probability be made to this
statement in favour of the singular genus Dictyonema (fig.
33), which is highly characteristic of the highest Cambrian beds
(Tremadoc Slates). This curious fossil occurs in the form of
fan-like or funnel-shaped expansions, composed of slightly-diverging
horny branches, which are united in a net-like manner by numerous
delicate cross-bars, and exhibit a row of little cups or cells, in
which the animals were contained, on each side. Dictyonema
has generally been referred to the Graptolites; but it
has a much greater affinity with the plant-like Sea-firs
(Sertularians) or the Sea-mosses (Polyzoa), and
the balance of evidence is perhaps in favour of placing it with
the latter.

LITERATURE.

The following are the more important and accessible works and
memoirs which may be consulted in studying the stratigraphical
and palæontological relations of the Cambrian Rocks:—

(1)

'Siluria.' Sir Roderick Murchison. 5th ed., pp. 21-46.

(2)

'Synopsis of the Classification of the British Palæozoic
Rocks.' Sedgwick. Introduction to the 3d Fasciculus of the
'Descriptions of British Palæozoic Fossils in the Woodwardian
Museum,' by F. M'Coy, pp. i-xcviii, 1855.

(3)

'Catalogue of the Cambrian and Silurian Fossils in the Geological
Museum of the University of Cambridge.' Salter. With a Preface
by Prof. Sedgwick. 1873.

"On the Tremadoc Rocks in the Neighbourhood of St David's,
South Wales, and their Fossil Contents." Hicks.—'Quart.
Journ. Geol. Soc.,' xxix. 39-52. 1873.

In the above list, allusion has necessarily been omitted to numerous
works and memoirs on the Cambrian deposits of Sweden and Norway,
Central Europe, Russia, Spain, and various parts of North America,
as well as to a number of important papers on the British Cambrian
strata by various well-known observers. Amongst these latter
may be mentioned memoirs by Prof. Phillips, and Messrs Salter,
Hicks, Belt, Plant, Homfray, Ash, Holl, &c.